Objective To evaluate effect of obesity on 12-month functional outcomes after surgery for lumbar stenosis in adult patients. Methods Data were collected on patients treated with lumbar laminectomy for symptomatic lumbar spinal stenosis as part of an observational registry and analyzed using a retrospective cohort study design. Patients with body mass index (BMI) >30 were compared with patients with BMI <30 with respect to baseline, 3-month, and 12-month functional status, adjusted for potential confounders. Results There were 101 patients. At baseline, patients with BMI >30 had significantly more back pain (P < 0.001), more leg pain (P < 0.001), lower EuroQol 5 dimensions questionnaire (EQ-5D) scores (P < 0.001), and higher Oswestry Disability Index (ODI) scores (P < 0.001). Both low- and high-BMI groups had significant improvement in back pain, leg pain, EQ-5D scores, and ODI scores after decompression (all P < 0.001). At 3 months postoperatively, high-BMI patients continued to report greater leg pain (P = 0.063) and higher ODI score (P = 0.064) relative to low-BMI patients. By 12 months, there was no difference between low- and high-BMI patients in back pain (P = 0.929), leg pain (P = 0.638), EQ-5D score (P = 0.733), or ODI score (P = 0.214). Conclusions The difference between low- and high-BMI patients trended toward significance for leg pain and ODI score at 3 months, but this difference disappeared by 12 months. This suggests that obese patients with symptomatic lumbar spinal stenosis may require longer to recover after decompression but can expect to reach equivalent outcomes of similarly treated patients with BMI <30.